Metal detectors are broadly categorized as either frequency-domain or time-domain, with the former being the dominant type in the hobby market, and the latter used for specialty markets such as mine detection and prospecting. Most frequency-domain detectors use a single fixed frequency and determine probable target type by the amount of phase shift incurred in the received signal. Besides an audible tone signifying a detected target, many detectors include a visual display which provides additional information on target identification (TID) and other parameters.
Fixed-segment or alphanumeric type LCD displays as shown in FIG. 1 allow metal detectors to visually display various information to the user. Typical information might include a numeric readout of the target's phase (a “visual display indicator” or “VDI” number) and a numeric readout of the estimated target depth, as well as certain user settings such as sensitivity. Often, instead of or in addition to the numeric readout, there is a segmented representation (usually in the form of a bar graph) of the same information.
However, fixed segment and alphanumeric displays are limited in both the amount of information that can be displayed and the format in which they can display it. For example, a fixed-segment display is designed to have elements (segments) which can only be turned on and off, and not repositioned or manipulated in any other way. Generally, if it is desirable to add any new information, the display must be redesigned. Alphanumeric displays are more limited in how much information they can show at any time and in how the information can be arranged, but they are more flexible in their ability to change the information being displayed. Since alphanumeric displays are non-custom devices, software changes are all that are needed to alter the displayed contents.
Bit-mapped graphical displays are becoming more popular in metal detectors, as they have in other consumer devices. Besides being a standard, non-custom display that can be fully manipulated in software, bit-mapped graphics add the ability to display more detailed information, real-time graphical waveforms, and allow for more user-friendly interfaces. U.S. Pat. No. 5,523,690 and U.S. Pat. No. 5,596,277 describe prior art of a graphical display. Besides displaying a VDI number and a probable target type, such a display can provide a detailed reading of phase response in a graphical plot, allowing the user to see how much of the target response correlates with the numerical VDI response. Some targets, such as iron, have scattered phase responses which are easily discerned in a graphical plot, but may produce a dominant VDI number which is misleading.
Frequency-domain detectors are increasingly moving toward the use of multiple frequencies, which increases the amount of information that can be presented on a visual display. As well, time-domain metal detectors are using multiple transmit sequences and multiple sample points to provide far more target information than earlier designs. A dramatic increase in microprocessor computing power has largely been responsible for these design evolutions, and this increase in computational power also creates the opportunity to perform additional kinds of analyses which further demand new methods of displaying target response information.